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There are relatively few well-documented reports about the mineralogy of Aridisols. A Durorthid and two Haplargids were sampled in the Mojave Desert, California. The soils formed in alluvial deposits estimated to be > 300 000, 200 000, and 100 000 yr old, respectively, and were examined for factors influencing variation in mineralogy of the clay, silt, and sand fractions. Smectite is the major constituent of the clay fraction in all three soils, whereas mica and kaolinite are less abundant. A broad 1.55-nm d spacing was most common in the Mg-saturated clay samples, indicating a low-charge and low-crystallinity smectite. The clay mineralogy of a buried soil underlying all three soils is also dominated by smectite, with minor amounts of kaolinite and mica. The similarity in clay mineralogy of these different-aged soils suggests that, even during wetter climates of the Pleistocene, leaching did not remove enough silica to create an unstable environment for smectite. The fine and medium silt fractions in all three soils are composed of quartz, feldspar, smectite, mica, kaolinite, and vermiculite. Treatment of the silt fraction with NaOH, which dissolves noncrystalline and short-range-ordered silica and aluminosilicates, released considerable smectite. This suggests that the smectite is composed of pedogenic microagglomerates and is not truly silt sized. A Mg-saturated x-ray diffraction maximum at about 1.9 nm, which decreased to about 1.6 nm after the NaOH treatment, was observed in some of the silt samples. The presence of the 1.9-nm peak is probably a result of noncrystalline aluminosilicates in the interlayers of smectite, rather than a result of interstratification of mica-smectite.